The two basic problems of immunizing against disease by vaccination are isolation of an antigen which is capable of provoking the production of protective antibodies, i.e. antibodies which inhibit or impair the ability of the disease causing organism to invade the host animal, and the presentation of this immunity provoking antigen to the hose animal in a way which actually stimulates production of the protective antibodies which confer immunity or partial immunity. Historically, the earliest efforts at immunization have involved the entire disease causing organism which was either killed or substantially weakened. In some cases the causative organism was not even separated from the medium in which it was obtained, the most famous example being the development of rabies vaccine by Pasteur. In many cases these "whole culture" vaccines caused undesirable side reactions. Thus more recent efforts have been directed toward isolating those molecules of the causative organisms which provoke the production of the appropriate antibodies.
This general development sequence was indeed followed in the case of vaccines against Streptococcus equi invasion of horses. The initial vaccine was comprised of killed whole cells but provoked undesirable side reactions which are detailed in the background discussion of U.S. Pat. No. 3,852,420. This patent is concerned with extracting an immunity provoking antigen by hot acid treatment of cells of this organism. However, this extract still contained extraneous materials which caused some undesirable side reactions such as swelling at the site of injection. U.S. Pat. No. 4,582,798 is concerned with an alternative technique for extracting the antigen of interest by treatment of the cells with mutanolysin and an anionic detergent. This technique yields an extract which causes an even further reduced incidence of side reactions.
The initial manner of presenting the antigenic material, whether the whole organism or an extract, was by introduction into the circulatory system of the host animal. Indeed it had been felt that the development of circulating antibodies, which was best accomplished by introduction of the antigenic material into the circulatory system, was critical to the development of immunity. Subsequently, it was found that in some cases immunity can be induced by localized introduction of antigenic material. For instance, the article "Protective Studies with a Group A Streptococcal M Protein II Challenge of Volunteers after Local Immunization in the Upper Respiratory Tract" by S. M. Polly, R. H. Waldman, P. High, M. K. Wittner, A. Dorfman and E. H. Fox at pages 217 to 224 of Volume 131, Number 3 (March 1975) of The Journal of Infectious Diseases reports on the immunization of humans against clinical illness induced by a class of microorganisms by the administration of an aerosol spray containing the appropriate antigen into the nose and oropharynx. In a follow up study the response to subsequent challenge by virulent organisms was compared to that of individuals vaccinated by subcutaneous injection and to that of unvaccinated individuals. The aerosol administration gave a measure of protection and provoked fewer and less severe side reactions than the parenteral injection route (see "Protective Studies with Group A Streptoccal M Protein Vaccine III Challenge of Volunteers after Systemic or Internal Immunization with Type 3 or Type 12 Group A Streptococcus") by R. D'Alessandri, G. Plotkin, R. M. Kluge, M. K. Wittner, F. N. Fox, A. Dorfman, and R. H. Waldman in The Journal of Infectious Diseases, Volume 138, number 6 (December 1978).
In the case of Streptococcus equi the presentation of the immunity provoking antigen has been by intramuscular injection of the target equines. In the case of the antigens extracted by acid or enzyme the injected vaccine has included an adjuvant to enhance the antibody response. These adjuvants were felt necessary to obtain a sufficient antibody response to reduce or suppress the symptoms on subsequent challenge with the virulent organism. It was felt that the slow release of antigen extract from the adjuvant into the blood stream of the vaccinated horse was critical to obtaining an adequate level of immunity.
A recent study has suggested that the antibodies generated locally at the site of invasion may be more significant than serum antibodies (those antibodies found generally in the circulatory system) for Streptococcus equi infections in the horse. In "Mucosal Nasopharyngeal Immune Response of Horses to Protein Antigens of Streptococcus equi" by J. E. Galon and J. F. Timeney at pages 623 to 628 of volume 47, number 3 (March 1985) of Infection and Immunity it is reported that the correlation between serum antibodies and field protection is rather poor and that locally generated antibodies recognized antigens not recognized by the serum antibodies. It is further hypothesized that neutralization of these antigens may be important in obtaining protection. However, intranasal vaccination was neither suggested nor attempted. Rather some ponies were given intramuscular injections of aluminum hydroxide adjuvanted acid extract of Streptococcus equi and then these vaccinates and unvaccinated control ponies were challenged by intranasal spraying of virulent organisms. All of the challenged ponies developed illness ("strangles") but were resistant to infection on rechallenge.
The intranasal vaccination of horses with killed bacteria (bacterins) or antigenic extracts presents special problems. It has not been established that such a route could actually provide any degree of immunity. Conventional wisdom has been that only a modified live vaccine could be effectively administered in this manner because only such a preparation would present an adequate concentration of antigen. Indeed some work with intranasal vaccination with influenza virus has been reported in U.S. Pat. No. 4,500,513. Furthermore, the delivery of the vaccine presents special problems because of the structure of the horse's nose. The vaccine should be delivered to the tonsilor tissue which is typically some 12 to 14 inches (30 to 35 cm) back from the nasal orifice. The normal technique for nasal application is by atomized spray as detailed in the D'Alessandri and Polly articles discussed hereinabove. However, delivery of an adequate dose into a horse's nose would require approximately five minutes and horses have virtually no tolerance for the hissing noise associated with such atomizers. It is believed that previous horse challenges such as that reported in the Galan article discussed hereinabove were conducted by sedating the animals before subjecting them to the atomized spray.